SIMPLEST NEURAL ARCS. 45 



absolutely inactive tissue is aroused to action by our stimulus. Rather, 

 we must think of the living central nervous system as in a continuous 

 state of excitation. In the waking state, at least, it probably originates 

 a continuous succession of centrifugal excitations, so that each "final 

 common path" has converging upon it every moment a complex of 

 stimulating and inhibiting impulses whose algebraic sum at any 

 moment of time conditions the state of the "final common path " at that 

 particular moment. A stimulus to reflex action is not a form of energy 

 to be transmitted to muscle. It does not develop activity in an other- 

 wise inert system. It merely modifies the balance of existing tendencies. 

 On these grounds the reflexes may not be expected to be uniform. 



The extreme susceptibility of the patellar reflex to peripheral re- 

 inforcement was shown by Jendrassik 1 in the familiar Kunstgriff; by 

 Weir Mitchell and Lewis 2 in simultaneous stimulation of the skin; 

 by Schreiber 3 in friction of the skin; by Beevor 4 in cold-water douches; 

 by Bowditch and Warren 5 through various methods; and by Stern- 

 berg 6 in the simple handclap. Similarly, central conditions of rein- 

 forcement and inhibition are in constant interplay. It is surprising- 

 how often in the literature of the patellar reflex one finds without a 

 sequel the "preliminary announcement" of some remarkable correla- 

 tion between the knee-jerk changes and various mental processes, like 

 attention. The verification of these supposed correlations seldom 

 appears. Only in spinal preparation are successive reflexes relatively 

 uniform. Of the various conditions that produce this lack of uniform- 

 ity only a few are definitely localizable like the specific action of curare, 

 strychnine, and carbolic acid. In general we know that reflex excita- 

 bility is modified by the degree of activity of the higher centers. Antag- 

 onistic and facilitating influences may also arise at or about the same 

 spinal level as the reflex itself. Variations in pulse-rate and blood- 

 pressure, and various phases of respiratory rhythm, also seem to modify 

 the reflexes. 



With a full realization of all these sources of variation, our first direct 

 and immediate problem is to discover whether the irritability of this 

 human reflex arc is increased or decreased by moderate doses of alcohol. 

 Assuming that all these sources of variation and many others may be 

 present in our records in greater or less degree, it was a technical 

 problem to equalize the conditions as far as practicable. The problem 

 of interpreting the results is first of all statistical. It is obvious that 

 the need of statistical treatment to eliminate as far as possible accidental 

 variations not otherwise shut out by our technique is just as great in 

 the simple as in the more complex processes. 



1 Jendrassik, Neurolog. Centralbl., 1885, 4, p. 412. 

 'Mitchell and Lewis, Med. News, 1886, Feb. 13, p. 48. 

 'Schreiber, Deutsch. Archiv f. klin. Med., 1884, 35, p. 254. 

 4 Beevor, Brain, 1883, 5, p. 56. 



'Bowditch and Warren, Journ. Physiol., 1890, 11, p. 25. 

 "Sternberg, Die Sehnenreflexe, Leipsic, 1893, p. 177. 



